false alarm conventional smoke in a h.v. room

we have a gent xenex panel with a ziton conventional smoke detector in place in an external h.v. room which has false alarmed about 5 times in a year (nobody told us there was a detector in the h.v. room so we've been looking in the wrong place!) we have changed the ziton smoke for another ziton smoke and it still false alarmed and we have also turned on the 'false alarm rejection' option on the xenex panel. no improvement.

I think it's the environment as the area is unheated and there is groundwater seeping into the h.v. room where the underground cables come in and the mesh on the air vents is so wide open plants can grow through! we have turned up the thermostat to try and dry out the room (and requested finer mesh on the vents to reduce insect ingress and requested a sump pump for the water but nothing gets done)

I was going to suggest an Apollo orbis multi detector to reduce false alarms. anyone got any better suggestions?

one of the site engineers keeps shouting about an ionisation detector but I told him these aren't made anymore for conventional panels. he then said 'well they do domestic ionisation detectors!' and I realised he's right they do. so why have people stopped making ionisation for conventional panels and yet domestic still use ionisation? Not that I'm going off on a tangent or anything...